Conventionally, a filter is provided for an exhaust gas passage in some cases in order to purify particulate matter (PM) discharged from an internal combustion engine. As for the filter, the ability to collect the particulate matter can be regenerated by oxidizing and removing the particulate matter accumulated in the filter by raising the temperature. However, the filter suffers from any failure or trouble in some cases, for example, due to the appearance of chipping and/or breakage and/or due to the occurrence of erosion or dissolved loss caused by the excessive increase in temperature when the regeneration process is performed as described above.
When the filter suffers from the failure (filter is out of order), the following possibility arises. That is, the amount of the particulate matter, which passes through the filter without being collected by the filter, may be increased, and the amount of the particulate matter, which is released to the outside of a vehicle, may be increased. In relation thereto, in recent years, the following filter failure diagnostics is known as one of On-Board-Diagnostics (OBD) performed by a computer carried on a vehicle. That is, the differential pressure between the upstream and the downstream of the filter is detected, and it is judged that any failure occurs if the differential pressure is abnormally small. However, in the case of the filter failure diagnostics described above, for example, the following inconvenience arises. That is, the particulate matter, which is accumulated in the filter, is oxidized on account of the influence of, for example, the temperature and NO2, the amount of accumulation of the particulate matter is changed, and it becomes difficult to grasp the abnormality of the filter by means of the differential pressure between the upstream and the downstream of the filter.
Another failure diagnostics, which utilizes a PM sensor for detecting the amount of particulate matter allowed to pass through a filter, has been suggested as another method for performing the failure diagnostics for the filter. The following technique is known in relation thereto. That is, those provided are a filter which is provided for an exhaust gas passage of an internal combustion engine and which collects particulate matter contained in an exhaust gas discharged from the internal combustion engine, and a PM sensor which is provided on a downstream side of the filter and which detects an amount of the particulate matter contained in the exhaust gas, wherein any failure of the filter is judged on the basis of the amount of the particulate matter which exists on the downstream from the filter and which is detected by the PM sensor. Further, in this procedure, the collection efficiency of the filter is taken into consideration (see, for example, Patent Document 1).